From dcf5aaafaff3c42922d6db5a53f32560cb6d8337 Mon Sep 17 00:00:00 2001
From: rachit-27 <bhatrachit29@gmail.com>
Date: Mon, 10 Nov 2025 12:45:02 +0530
Subject: [PATCH 1/3] Add Shell Sort algorithm (fixes #13887)

---
 sorts/shell_sort.py            | 65 ++++++++++++++++++++--------------
 tests/sorts/test_shell_sort.py | 21 +++++++++++
 2 files changed, 59 insertions(+), 27 deletions(-)
 create mode 100644 tests/sorts/test_shell_sort.py

diff --git a/sorts/shell_sort.py b/sorts/shell_sort.py
index b65609c974b7..9aa234cfa3ae 100644
--- a/sorts/shell_sort.py
+++ b/sorts/shell_sort.py
@@ -1,40 +1,51 @@
 """
-https://en.wikipedia.org/wiki/Shellsort#Pseudocode
+Shell Sort Algorithm
+--------------------
+
+Issue: #13887
+Implements the Shell Sort algorithm which is a generalization of insertion sort.
+It improves by comparing elements far apart, then reducing the gap between elements
+to be compared until the list is fully sorted.
+
+Time Complexity:
+    Worst case: O(n^2)
+    Best case:  O(n log n)
+    Average:    O(n^(3/2))
+
+Space Complexity: O(1)
 """
 
+from __future__ import annotations
 
-def shell_sort(collection: list[int]) -> list[int]:
-    """Pure implementation of shell sort algorithm in Python
-    :param collection:  Some mutable ordered collection with heterogeneous
-    comparable items inside
-    :return:  the same collection ordered by ascending
 
-    >>> shell_sort([0, 5, 3, 2, 2])
-    [0, 2, 2, 3, 5]
+def shell_sort(arr: list[int]) -> list[int]:
+    """
+    Sorts the given list using Shell Sort and returns the sorted list.
+
+    >>> shell_sort([5, 2, 9, 1])
+    [1, 2, 5, 9]
     >>> shell_sort([])
     []
-    >>> shell_sort([-2, -5, -45])
-    [-45, -5, -2]
+    >>> shell_sort([3])
+    [3]
+    >>> shell_sort([1, 2, 3])
+    [1, 2, 3]
+    >>> shell_sort([4, 3, 3, 1])
+    [1, 3, 3, 4]
     """
-    # Marcin Ciura's gap sequence
+    n = len(arr)
+    gap = n // 2
 
-    gaps = [701, 301, 132, 57, 23, 10, 4, 1]
-    for gap in gaps:
-        for i in range(gap, len(collection)):
-            insert_value = collection[i]
+    # Keep reducing the gap until it becomes 0
+    while gap > 0:
+        for i in range(gap, n):
+            temp = arr[i]
             j = i
-            while j >= gap and collection[j - gap] > insert_value:
-                collection[j] = collection[j - gap]
+            while j >= gap and arr[j - gap] > temp:
+                arr[j] = arr[j - gap]
                 j -= gap
-            if j != i:
-                collection[j] = insert_value
-    return collection
-
+            arr[j] = temp
+        gap //= 2
 
-if __name__ == "__main__":
-    from doctest import testmod
+    return arr
 
-    testmod()
-    user_input = input("Enter numbers separated by a comma:\n").strip()
-    unsorted = [int(item) for item in user_input.split(",")]
-    print(shell_sort(unsorted))
diff --git a/tests/sorts/test_shell_sort.py b/tests/sorts/test_shell_sort.py
new file mode 100644
index 000000000000..9dc612ff2741
--- /dev/null
+++ b/tests/sorts/test_shell_sort.py
@@ -0,0 +1,21 @@
+from sorts.shell_sort import shell_sort
+
+
+def test_shell_sort_basic():
+    assert shell_sort([5, 2, 9, 1]) == [1, 2, 5, 9]
+
+
+def test_shell_sort_empty():
+    assert shell_sort([]) == []
+
+
+def test_shell_sort_one_element():
+    assert shell_sort([3]) == [3]
+
+
+def test_shell_sort_sorted():
+    assert shell_sort([1, 2, 3, 4]) == [1, 2, 3, 4]
+
+
+def test_shell_sort_duplicates():
+    assert shell_sort([4, 3, 3, 1]) == [1, 3, 3, 4]

From 067a10067791f0ff8bd25aaa3e5a4b98d5106064 Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Mon, 10 Nov 2025 07:19:21 +0000
Subject: [PATCH 2/3] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 sorts/shell_sort.py | 1 -
 1 file changed, 1 deletion(-)

diff --git a/sorts/shell_sort.py b/sorts/shell_sort.py
index 9aa234cfa3ae..43eee0b8d090 100644
--- a/sorts/shell_sort.py
+++ b/sorts/shell_sort.py
@@ -48,4 +48,3 @@ def shell_sort(arr: list[int]) -> list[int]:
         gap //= 2
 
     return arr
-

From fe07cc00dbe1e8075a7e78586efdb7707b9f2a42 Mon Sep 17 00:00:00 2001
From: rachit-27 <bhatrachit29@gmail.com>
Date: Mon, 10 Nov 2025 12:55:06 +0530
Subject: [PATCH 3/3] Fix: pass Ruff and pre-commit checks for Shell Sort
 (#13887)

---
 tests/__init__.py       | 0
 tests/sorts/__init__.py | 0
 2 files changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 tests/__init__.py
 create mode 100644 tests/sorts/__init__.py

diff --git a/tests/__init__.py b/tests/__init__.py
new file mode 100644
index 000000000000..e69de29bb2d1
diff --git a/tests/sorts/__init__.py b/tests/sorts/__init__.py
new file mode 100644
index 000000000000..e69de29bb2d1